Chemistry of carbanions. XVIII. Preparation of trimethylsilyl enol ethers

Abstract

Two procedures for the preparation of trimethylsilyl enol ethers from aldehydes and ketones are described. Reaction of the ketone with chlorotrimethylsilane and triethylamine in dimethylformamide solution usually affords an equilibrium mixture of the trimethylsilyl enol ethers. Successive reaction of the ketone with lithium diisopropylamide and with chlorotrimethyhilane in l12-dimethoxyethane solution normally produces a mixture in which the less highly substituted enol ether (from kinetically controlled enolate formation) is the principal product. A number of representative structurally and stereochemically isomeric trimethylsilyl enol ethers have been characterized (see Table 11) and their physical properties have been studied. (1968). (5) For descriptions of the preparation of silyl enol ethers, see (a) C. R. Krager and E. G . Rochow, J . Orwnometal. Chem., 1, 476 (1964); (b) J. I. Baukov, G . S. Burlachenko, and I. F. Lutsenko, ibid., 8, 478 (1965); (c) R. J. Fessenden and J. S. Feesenden, J . Ow. Chem., 82, 3535 (1967); (d) R. Bourhis and E. Frainnet, Bull. Soe. Chim. Fr., 3552 (1967); (e) U. Schmidt and M . Schwochan, Tetrahedron Lett., 4491 (1967); (f) G. Stork and P. F. Hudrlik, J . Amer. Chem. Soc., 90, 4462, 4464 (1968). (6) For examples of the rearrangement of 2-silyl ketones to enol ethers, see (a) A. G . Brook, D . M. MacRae, and W. W.Limburg, J . Amer. Chem. Soc., 89, 5493 (1967); (b) I. F. Lutsenko, Yu. I. Baukov, 0. V. Dudukina, and E. N. Kramarova, J . Organometal. Chem., 11, 35 (1968). (8) The use of trimethylsilyl chloride as a scavenger for alkoxides has been used to advantage in the acyloin condensation. See (a) J. J. Bloomfield, Tetrahedron Lett., 587, 591 (1968); (b) G. E. Gream and 9. Worthley, ibid., 3318 (1868).

Reagents and synthetic methods. 31. Silylations with N-trimethylsilyl-2-oxazolidinone (TMSO)

“…The reaction takes place rapidly, 15-30 min, at room temperature and the desired silyl en01 ethers are obtained in high yield. It should be noted that the thermodynamic-to-kinetic isomer ratio for methylcyclohexanone, 89 : 1 1, was different from the ratio 78 : 22, reported by House et al, with triethylamine in refluxing dimethylformamide (10). This seems to be due to the difference in reaction conditions.…”

The Enantioselective Tsuji Allylation

“…The minor isomer was removed by fractional distillation with a spinning band column. 5 1 H NMR (300 MHz, CDCl 3 ) δ 2.00 (m, 2H), 1.94 (m, 2H), 1.64 (m, 2H), 1.58-1.49 (m, 5H), 0.16 (s, 9H); 13 General Procedures for the Synthesis of Allyl Enol Carbonates.…”

Curiosidades sobre a reação aldólica utilizada como etapa chave na síntese Brasileira dos ácidos pterídicos A e B

“…19 Os autores investigaram vários outros silil enol éteres e apontaram que o hidrogênio vinílico em um diastereoisômero Z, geralmente, aparece em menor deslocamento químico (mais protegido) que o correspondente em um diastereoisômero E. 19 A perda de seletividade, neste caso, pode ter sido ocasionada pelo favorecimento da enolização Z-seletiva em HMPA. Seguindo uma linha lógica, repetimos a reação nas mesmas condições de enolização da reação aldólica utilizada na síntese dos ácidos pterídicos (Esquema 6, condição reacional III, Tabela 4).…”

“…19 Provavelmente, silil enol éteres como estes, os quais apresentam uma porção dieno conjugado, possuam características muito peculiares e diferentes daqueles estudados por House (Material suplementar, Figura 13S).…”